Methods of making electrical resistors



April 4, 1967 J. J. H. ARDOUIN ET AL 3,311,968

METHODS OF MAKING ELECTRICAL RESISTORS Filed Dec. 5, 1962 2 Sheets-Sheet 1 Z V '9 'IIIIIII!IIIIMIIWIWMIW n April 4, 1967 J. J. H- ARDOUIN ET 3,311,968

METHODS OF MAKING ELECTRICAL RESISTORS Filed Dec. 5, 1962 2 Sheets-Sheet 2 United States Patent 7 Claims. ci. 29-1559) The present invention relates to coated electrical resistors constituted by small rods or elongated cores of substantially cylindrical form of an insulating material covered at least partly with an electrically conductive coating and provided at their ends with conductive terminal caps connected electrically with said coating, and to methods of making such resistors.

The chief object of our invention is to provide a resistor and a method of manufacturing it which respond better than hitherto to the various requirements of practice, especially with regard to the strength of mechanical fixation and to the firmness of the electrical contact between the resistor and its caps.

The resistor according to this invention comprises, in combination, an elongated core element of a non conductive material having its end portions provided with grooves, respectively, said core element being provided, between said end portions, with at least one groove ending at its respective ends in said first mentioned grooves respectively, a conductor housed in said second mentioned groove, non conductive means for protecting said conductor in said second mentioned groove, a resilient cap tightly applied on each of said end portions, said cap being made of a material which is a good conductor of electricity, means in said first mentioned grooves for electrically connecting the ends of said conductor with said caps, respectivel and electric terminals carried by said caps, respectively. p

The method according to our invention for the manufacture of an electric resistor consisting of a small cylindrical core of an insulating material covered at least partly with an electrically conductive coating and provided at its ends with conductive caps connected electrically with said coating comprises providing at least one of the axial ends of the core with a series of grooves and ribs, covering this core end with an electrically conductive film connected electrically with said conductive coating, and force fitting around the core end covered with said conductive film a hollow end cap the thinand deformable conductive wall of which has a widened opening and the internal diameter of which is less than the maximum diameter of said core end to be capped.

Preferred embodiments of the invention will be hereinafter described with reference to the appended drawings, given merely by way of example, and in which:

FIGS. 1 and 2 show respectively two cores of insulating material intended to be used in the manufacture of resistors according to the invention;

FIGS. 3 to 6 are sectional views illustrating different stages of the manufacture of a resistor obtained from the core of FIG. 1;

FIG. 7 shows a terminal cap alone, in axial section;

FIGS. 8 to 10 show three resistors equipped with their caps, made according to the invention. 1

FIG. 11 shows in elevational view another core of insulating material to be used in the manufacture of a resistor according to the invention;

FIG. 12 is a perspective view of a core provided with longitudinal grooves to be used for manufacturing a resistor, according to another embodiment of the invention;

FIG. 13 is a middle cross sectional view of a resistor made on a core according to FIG. 12 during a step of its manufacture;

FIGS. 14-15 and 16 are respectively an axial section of FIG. 13 and cross sections on the lines XV-XV and XVI-XVI, respectively, of FIG. 14 of the finished resistor.

Before coming to the heart of the subject, it should be reminded that it is known to make resistors by depositing a thin coating of a conductive material (carbon or a metallic alloy) of high resistivity upon an insulating body of general cylindrical shape having a threaded or smooth surface, then to transform this deposit into a resistant helix either by abrading the crests of the threaded surface or by cutting a helical groove into the smooth surface.

In order to be able to make use of such a resistor, it is of course necessary to connect its ends to output condu-ctors with the aid of sturdy contacts, without introducing any variation in the ohmic value, nor any thermoelectric couple, nor a had contact likely to cause interference noises in the electrical transmissions.

In order to effect these contacts, one operated hitherto:

Either by screwing threaded caps on the ends of the threaded core element (first case above considered);

Or by force fitting caps over the ends of the smooth core element (second case).

In both cases, the quality of the contact between the cap carrying the output conductor and the conductor coating is conditioned by the tight adjustment of :the cap on this coating.

Now this adjustment varies not only with the difference between the internal diameter of the cap and the external diameter of the insulating core element, but also with the temperature, because the respective coefiicients of thermal expansion of the cap and of the core element are generally different; if, for example, the coefficient of expansion of the cap is greater than that of the insulating material, a rise in temperature results in the formation of a clearance, and thus of a poor contact between the cap and the conductive coating, whereas a lowering in temperature causes stresses which may cause the cap to burst.

In order to palliate these draw-backs and to ensure an excellent mechanical and electrical contact between the cap and the conductive coating at all temperatures, the ends of the core element having an external surfacewhich is threaded or formed with a series of and ribs, this surface is covered with a conductive metallic deposit which is connected electrically with, or forms part of, the resistant coating, and over the surface portion thus metallised there is force fitted a conductive cap which is itself connected or easily connectable with an external conductor wire.

FIG. 1 shows an insulating core element 1 the whole external surface of which is cut with a continuous helical thread 2.

On the other hand, the insulating core element 3 of FIG. 2 is a smooth rod only the ends of which are cut with annular grooves 4 separated from one another by ribs 5.

The axial sections of thread 2 and of grooves 4 may of course have any desirable shape, such as triangular, trapezoidal, rectangular, rounded, etc.

As the method for securing the caps: is the same for these two types of core element, it is considered sufficient to describe it hereinafter as applied to the type illustrated by FIG. 1.

The end of this core element 1 is first covered with a metallic film 6 (FIG. 3) of high conductivity, for example by depositing upon said end, by means of a brush, for instance a rotary one, a solution containing a conductive alloy or metal (such as silver) in the colloidal state or by projecting a conductive metal or alloy in the annular grooves fused state under high pressure onto said end, or in any other manner.

In order to effect the electrical connection between this film 6 and the resist-ant coating 7 (of carbon or alloy) intended to form the resistor, the film can be extended until it covers said resistant coating, if the latter is already in place. But it is preferred to operate as follows, in order to avoid the risks of oxidation of this coating 7.

The metallised end of core element 1 is covered with a cap (FIG. 4), in such fashion that a portion of film 6 extends out beyond this cap 8.

Then coating 7, of high resistivity, is deposited on the core element fitted with said cap 8, especially by vaporisation in a vacuum if said coating 7 is a metal or alloy such as chromium-nickel alloy.

The portion of this coating 7 deposited on the portion of film 6 which extends out beyond cap 8 effects the electrical connection between film 6 and coating 7.

Then the coating 7 is covered, by vaporisation in a vacuum, with a protective film 9 of silicon monoxide SiO.

It is then possible, without risk of oxidation, to remove the core element from the vacuum vaporisation bell and advantageously complete its covering with another protective coating 10 (FIG. which fills in the hollows of the screw threads and has a smooth cylindrical external surface, as described in French Patent No. 1,209,074 filed Mar. 19, 1958.

This coating is preferably constituted by an insulating material capable of hardening on baking, such as a ceramic, a glass, a thermosetting or other resin, which material may or may not be identical with that of which core element 1 is constituted.

Cap 8 is then withdrawn.

It is then advantageous, although not essential, to fit in the hollow of conductive thread .6 a cord 11 of conductive and malleable material having a low melting point, for example of lea-d-tin alloy, the cross-section of this cord preferably being such that it fills the groove of the thread as Well as possible without projecting beyond the crest thereof. The resistance in FIG. 5 has been represented in this phase of its manufacture.

Then a hollow cap 12 (FIG. 7) having for example the form of a cylindrical thimb'le with a flaring opening, is force fitted (FIG. 7) onto the thus prepared end of core element 1, this cap 12 being made of a malleable talloy or metal of good electrical conductivity (brass, copper, silver, etc), the internal diameter of tits cylindrical Wall being between the groove bottom diameter and the crest apex diameter of the end to be capped.

If, the local temporary expansions experienced by the wall of this cap 12 when it passes over the crests of the threads do not exceed the limit of elasticity of the constituentm-aterial of said wall, this cap tends to reassurne its original diameter in every groove, at the end of the force fitting operation (FIG. 6). We thus obtain a very great pressure of said cap 12 upon the crests of the threads and its partial introduction into the grooves thereof, ensuring on the one hand a very strong securing of the cap on the core element and on the other hand an excellent electrical contact between this core element and conductive film 6. Furthermore if a cord of solder 11 has been lodged in the grooves of the threads, the radial contraction of the wall of the cap 12 at the end of its fitting tends to crush this cord, causing it to adopt the shape of its lodgement (FIG. 6).

After fitting, it is advantageous to bring the assembly to a sufficiently high temperature (for example towards 300 C.), either in a vacuum or not, in order to weld the various superimposed elements to one another, and in particular:

Cap 12 on film 6, especially through the crushed cord 11, which then is softened or even melted, if use is made of such a cord;

Coating 7 upon the film portion 6 which it covers;

Protective coating 10, if it is used, upon core element 1, through the pores of the conductive coating 7 and of film 9.

After this baking, the roughed-out resistance obtained constitutes a single block rod (the external surface of which is smooth and hard if the hardenable coating 10 above has been provided) Within which there is provided a continuous and threaded conductor connected electrically to the two end caps.

This roughed-out piece is then abraded in any desirable fashion, for example by means of a milling cutter, until at least partial removal of the crests of the embedded conductor, this abrasion operation being carried out in conjunction with a permanent measurement of the resistance of said piece and halted when this resistance has reached the desired value.

It should be noted that, in the case of the use of the above-mentioned hardenable coating 10, the external surface of the roughed-out resistance remains permanently smooth and hard before, during and after the abrasion.

Then on the resistance obtained there is deposited a coating 13 (FIG. 6) of varnish or paint, in order to complete the protection thereof, although this is not essential especially in the case where the above mentioned h-ardenable coating 10 is used, in which case the very slight thickness of conductive coating 7 the edge of which comes to the surface of the abraded rod is immersed in a very hard material.

Every output wire 14 is generally simply fixed by soldering or in any other manner upon the corresponding cap 12, either axially (FIG. 8), or radially (FIG. 9), but it can further be firmly attached to the core element itself, for example by imprisonment of its end in an axial hole 15 of the said core element with the aid of a sealing cement 16 (FIG. 6) which sets during the above mentioned heating. In such a case it is then sufiicient to solder the cap 12 on the wire 14 (which passes through it axially) with the aid of a drop of conductive solder 17, to ensure the electrical connection between these two elements.

According to another modification, the two conductive outlet wires 14 are caused to come out at the same end of the resistor, one of these two wires passing axially through the entire length of the core element (FIG. 10), in a recess such as 15 above mentioned.

Naturally in such a case, recess 15 can be filled in, after introduction of wire 14, with an insulating substance, advantageously of the same nature as that of the core element so as to fuse intimately therewith on subsequent baking as above discussed.

Consequently, and irrespective of the mode of embodiment adopted, finally resistances are available for each of which the end caps are practically impossible to remove and are in intimate and durable electric contact with the actual resistance portion.

By way purely of illustration and not of limitation of the invention, it is indicated that in a form of embodiment thereof which has given full satisfaction, core element 1 was made of a ceramic material, as also coating 10, the pitch of the threads was 0.7 mm. and its maximum diameter was 4.25 mm., film 7 was formed by deposition, at ordinary temperature, of the solution known under the name liquid silver, coating 6 was a chromiumnickel alloy with 20% chromium by weight, vaporised under extreme vacuum, preferably below 10 mm. Hg and had a thickness of 0.1 micron, turns 11 had a diameter of 0.5 mm. and were constituted by a lead-tin alloy, cap 12 were made of brass and had an axial length of approximately 3 mm., an internal diameter of 4.12 mm. and a wall thickness of 0.2 mm. and the values of the resistances obtained before abrasion of the crests of the threads were between 50 and 200 ohms (for the said thickness of 0.1 micron); after abrasion of said crests,

said resistance values were generally multiplied by a coefiicient between 1 and 30 or more.

As is self-evident and as furthermore already appears from the above, the invention includes all possible modifications thereof, especially:

Those where the actual resistance portion is obtained by the cutting of a groove in a cylindrical covering (FIG. 2), and not by abrasion of the crests of a threaded covering;

Those where conductive film 6 in contact with the cap is constituted by a mere extension of the resistant coating 7;

Those Where coating 10 and/or turns 11 and/or the heating treatment are not used;

Those where said coating 10 is applied on the roughedout resistance after mounting of the caps and covers the latter at least partly;

Those where the abrasion of the crests (inthe case of the use of a completely threaded core element) is effected before the mounting of the caps, the roughed-out resistance then being held by its two metallised ends;

Those where the caps are constituted by mere eyelets open at both axial ends, and not by thimbles having a bottom;

Those where said caps have a constricted internal surface of variable diameter, and not cylindrical, the smallest diameter of this surface being less than the external diameter of the metallised end to be capped.

The core to be used according to the invention and which in the preceding example was provided with the simple thread may be also provided with a multiple thread.

FIG. 11 shows such a core where the helical thread is a triple thread 116-117-118 instead of the simple thread 2 of FIG. 1.

In this case, if n is the multiplicity of the thread (it being equal to three in the example of FIG. 11) the ohmic value of the resistance is n times smaller than in the case of a simple thread and the induction is also divided by n which is advantageous in many applications where alternating current is used.

The above described construction is quite satisfactory for the manufacture of resistors of average or high ohmic value. 1

But in the case of resistors of low ohmic value (for instance lower than one hundred ohms) this construction is not suitable, especially if these resistors are to be used in circuits for high frequency alternating currents.

As a matter of fact, in order to permit an adjustment of the value of the resistance which is not too delicate, the resistor must include a minimum number of spires, for instance five. The induction coefficient L of the resistor can therefore not become lower than a given minimum. Consequently, if the value R of the resistance is small and if an alternating current of high frequency f is to flow through the resistor, the phase shift (p due to this resistor (defined by the relation: tg =21rf. L/R) becomes prohibitive.

In order to obviate this drawback, according to an embodiment of the invention, the core is provided with longitudinal grooves and this core is treated in the same manner as above described. We thus obtain a resistor the active portion of which consists not of a helical conductor but of parallel bars electrically connected in shunt fashion.

In this case the initial core is of the shape of a ribbed cylinder, that is to say a cylinder provided with longitudinal grooves 2a separated from one another by ridges 30. The cross section of these grooves may be of any desired shape, for instance in the shape of a triangle, a trapezium, a rectangle, a line of rounded shape and so on.

This core is then treated in a manner analogous to that above described in the case of a core provided with helical grooves.

:In this case the cord 11 visible on FIG. of the preceding embodiment is replaced by a plurality of conducting elements 111 housed in the grooves provided at the ends of the core, these elements 111 being for instance made of a welding material for instance a lead-tin alloy. These elements are, in particular, in the form of grains, of small prisms or of small cylinders which fill the grooves without projecting beyond the crest thereof.

When the caps 112 are forced on the ends of the resistor the portions of these caps which are in contact with the ridges 30 of core 2a are caused to expand, but the resiliency of said elements 112 permits this expansion so that the side wall of a cap 112 is strongly applied against said ridges thus ensuring on the one hand a very strong fixation of the cap on the core and on the other hand a very good electric contact between this cap and the conducting film 106.

The portions of the cap which are located opposite the grooves of the core tend to keep their initial diameter while somewhat crushing elements 111, if such elements are used, so, that said elements are strongly applied to their housings and have an improved contact both with film 106 and with the cap 112.

When the ridges of the core are cut subsequently during the abrasion operation, the conductor layer is divided into a plurality of parallel bars 7 housed in the bottoms of grooves 2a. These bars are mounted in parallel between caps 112 so that the resistor can easily be given a low value and adjusted accurately.

It is important to point out that due to the parallellism of these bars, the resistor that is obtained has an induction coefiicient which is practically zero, so that it can be used in circuits through which pass alternating currents of very high frequency (for instance several megacycles).

By way of example of an embodiment of the invention, having no limitative character, the ribbed core 1a was made of ceramic material, same as layer 10. It had a diameter of 4.25 mm. at the outer ridges of the grooves and a length of about 15 mm. The number of grooves was twelve and their transverse section was an isosceles rectangular triangle so that the depth thereof was about 0.6 mm. The conductor film 106 consisted of a deposit at ordinary temperature of the solution known as liquid silver. Layer 7 was made :of a chromium and nickel alloy containing 20% by weight of chromium vaporized under a high vacuum that is to say preferably under a pressure lower than 10 mm. of Hg. and its thickness was 0.1 micron. Elements 111 consisted of wire portions of a lead-tin alloy. The caps 112 were made of brass and had an axial length of about 3 mm., an. inner diameter of 4.12 mm. and a wall thickness of 0.2 mm. The values of the resistors thus obtained were from 25 to ohms before abrasion. After abrasion of the crests the resistors, adjusted with a high accuracy, ranged from 30 to 200 ohms.

In a general manner While the above description, discloses what are deemed to be practical and efficient embodiments of the invention, the present invention is not limited thereto as there might be changes made in the arrangement, disposition and form of the parts Without departing from the principle of the invention as comprehended within the scope of the appended claims.

What we claim is:

1. The method of manufacturing an electric resistor which comprises starting from a cylindrical core of an insulating material, providing each of the axial ends of the core with a series of grooves, covering each of said core ends with an electrically conductive film, forming on said core an electrically resistive coating connected electrically with said end conductive films, and force fitting on each of said core ends, in such manner as partially to interfit said grooves, a hollow end cap of a substance having a good electrical conductivity the internal diameter of which cap is slightly less than the maximum diameter of said core end to be capped.

2. A method according to claim 1, wherein said film '7 and said cap are made of fusible metals and, after the fitting of said cap, the assembly is heated to a sufficient temperature to perform a fusion bonding of the cap with the conductive film covering the end in question.

3. A method according to claim 1, which further comprises, before the fitting of the hollow cap, placing in at least a portion of the grooves covered with the conductive film a conductive and malleable material having a low melting point and distinct from said resistant coating and said conductive film and fusing said material.

4. The method of manufacturing an electric resistor which comprises starting from a cylindrical core of an insulating material, providing each of the axial ends of the core with a series of grooves, covering each of said core ends with an electrically conductive film, fitting a protective cap on each of said core ends, While leaving uncovered the respective inner end portions of said conductive films, vaporizing a thin layer of a conductive substance in a vacuum onto said core and said inner end portions of said films which extend out beyond said protective caps so as to form on said core an electrically resistive coating connected electrically with said conductive films, covering said resistive coating with a protective film by vaporization in a vacuum, removing said protective caps from said core ends and force fitting on each of said core ends, in such manner as partially to interfit said grooves, a hollow end cap of a substance having a good electrical conductivity the internal diameter of which cap is slightly less than the maximum diameter of said core end to be capped.

5. A method according to claim 1 wherein the core of insulating material covered with said resistive coating E5 is covered with a coating of :an insulating material capable of hardening by baking, and the whole is subjected to a baking treatment for hardening it.

6. A method according to claim 5 which further corn,- prises, before the baking treatment, subjecting the core covered with said coating of an insulating material to an abrasion treatment to give it an external cylindrical shape.

7. A method according to claim 5, wherein the coating of insulating material capable of hardening by baking is applied after mounting of the end cap so as to cover said cap.

References Cited by the Examiner UNITED STATES PATENTS 1,767,715 6/1930 StOekle 29155.7 1,832,466 11/1931 Means 338-329 X 1,847,653 3/1932 Jones et al 29-1557 1,859,112 5/1932 Silberstein 29155.7 2,281,843 5/1942 Jira 29-155.7 2,400,404 5/1946 Fruth 2915S.7 2,416,347 2/ 1947 Rector 338300 X 2,508,511 5/ 1950 Goodman 338-266 2,682,595 6/1954 Rubinstein 338-266 2,792,620 5/1957 Kohl'ing 29155.7 2,803,054 8/1957 Kohring 29-155.7 3,060,063 10/1962 Bickford 29-155.7

JOHN F. CAMPBELL, Primary Examiner.

RICHARD M. WORD, Examiner. V. Y. MAYEWSKY, J. D. BOCK, Assistant Examiners. 

1. THE METHOD OF MANUFACTURING AN ELECTRIC RESISTOR WHICH COMPRISES STARTING FROM A CYLINDRICAL CORE OF AN INSULATING MATERIAL, PROVIDING EACH OF THE AXIAL ENDS OF THE CORE WITH A SERIES OF GROOVES, COVERING EACH OF SAID CORE ENDS WITH AN ELECTRICALLY CONDUCTIVE FILM, FORMING ON SAID CORE AN ELECTRICALLY RESISTIVE COATING CONNECTED ELECTRICALLY WITH SAID END CONDUCTIVE FILMS, AND FORCE FITTING ON EACH OF SAID CORE ENDS, IN SUCH MANNER AS PARTIALLY TO INTERFIT SAID GROOVES, A HOLLOW END CAP OF A SUBSTANCE HAVING A GOOD ELECTRICAL CONDUCTIVITY THE INTERNAL DIAMETER OF WHICH CAP IS SLIGHTLY LESS THAN THE MAXIMUM DIAMETER OF SAID CORE END TO BE CAPPED. 